Bent electro-absorption modulator

ABSTRACT

The present invention relates to a method and a device for modulating optical signals based on modulating bending losses in bend, quantum well semiconductor waveguide sections. The complex refractive index of the optical active semiconducting components of the waveguide section is modulated by applying a variable electric or electromagnetic field. The modulation of the complex refractive index results in a modulation of the refractive index contrast and the absorption coefficient for the waveguide at the frequency of the light. By carefully adjusting the composition of the semiconducting components and the applied electric field in relation to the frequency of the modulated radiation, the bending losses (and possibly coupling losses) will provide extinction of light guided by the bent waveguide section. The refractive index contrast may be modulated while keeping the absorption coefficient substantially constant and small, whereby the guided light can be modulated only by bending losses. Alternatively, the invention may be applied to enhance the extinction ratio of existing absorption modulators such as Electro-Absorption Modulators (EAMs) in which case extinction by absorption and extinction by bending losses co-operates to provide more compact modulators with improved performance (extinction) and speed.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and a device formodulating optical signals based on modulation of the absorption and ofthe bending losses in bend, quantum well semiconductor waveguidesections. The complex refractive index of the optical activesemiconducting components of the waveguide section is modulated throughthe Quantum Confined Stark Effect (QCSE), by applying a variableelectric or electromagnetic (EM) field. The modulation results in amodulation of the effective refractive index contrast and the absorptioncoefficient for the waveguide at the frequency of theoretical signal.

BACKGROUND OF THE INVENTION

[0002] In optical communication it is often of interest to obtain a highbit rate in the optical signals, the improvement of the presentstandards of 10 Gbit/s being restrained by the modulation speed ofoptical modulators. Typically, two classes of optical modulators areused, interferometric devices such as Mach-Zehnder type modulators andElectro-Absorption Modulators (EAMs).

[0003] Mach-Zehnder modulators utilise optic active materials to controla phase shift between two arms in an interferometer whereby theresulting signal may be modulated. Mach-Zehnder modulators presentlyprovide modulation speeds up to 40 Gbit/s, however, 100 Gbit/s have beenreported. It is a disadvantage of Mach-Zehnder modulators that they aretypically large, expensive, and require a large voltage amplitude toproduce the required phase shift.

[0004] In EAMs, a modulated absorption coefficient is induced in activesemiconductor materials using a modulated electric field, i.e. utilisingQCSE. There are two characteristic energy regimes for a semiconductormaterial, being denoted as below bandgap and above bandgap, where theabsorption coefficient (proportional to the imaginary part of therefractive index) of the semiconductor material is zero or non-zero,respectively. This is shown schematically in FIG. 1 where the curvesshow no absorption at low energies/frequencies below bandgap and highabsorption at high energies/frequencies above bandgap. The boundarybetween these two regimes, i.e. the bandgap region where the curves risesteeply, can be shifted due to the Quantum Confined Stark Effect (QCSE)when the material comprises a Quantum Well semiconductor structure. Thisis also shown in FIG. 1, where the absorption (i.e. the imaginary partof refractive index) are shifted to lower energies/lower frequencieswhen a reverse bias is applied. The QCSE in bulk structures is denotedthe Franz-Keldysh Effect (FKE). The QCSE is observed when reversebiasing the semiconductor structure. The amount of absorption near thebandgap is thereby increased for increasing reverse bias. Thereby, theoptical absorption may be modulated between a low and a high value forlight in a narrow energy/frequency region as indicated by the shadowedregion 2 in FIG. 1. The change in the absorption due to the QCSE or FKEis the mechanism used in EAMs. Presently, EAMs can provide modulationsspeeds up to 40 Gbit/s.

[0005] In an article by Veldhuis et al, Optics Communications, 168(1999) 481, an optic intensity modulator based on a bent channelwaveguide is disclosed. The bent channel waveguide has a fixed bendingradius. When the lateral refractive index contrast between the core andthe cladding material is high enough, all the light in the waveguidewill be guided. If the lateral refractive index contrast is loweredsufficiently, part of the light will be radiated out of the waveguide,the exact fraction depending on the value of the contrast. By adjustingthe contrast, the precise transmitted power may be controlled. FIG. 6summarises the length and changes ∃n_(act) in the refractive index ofthe core, assuming constant cladding index, required to achieve a 30 dBextinction. Veldhuis et al proposes the use of thermo-optic orelectro-optic actuation for controlling the refractive index inthermo-optic or electro-optic polymers applied in N×M matrix switches todecrease cross-talk and increase compactness.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a device anda method for modulating electromagnetic (EM) radiation, which provide anenhanced extinction ratio and faster operation than conventionalmodulators.

[0007] It is another object of the present invention to provide a deviceand a method for modulating EM radiation, which provides more compactmodulators than conventional modulators.

[0008] It is a further object of the present invention to provide adevice and a method for modulating EM radiation, which requires smallervoltage swings than conventional modulators.

[0009] The response of an optical active semiconductor material to an EMfield (light) is governed by the complex refractive index of thesemiconductor material, generally denoted as n=Re(n)+iIm(n). Theimaginary part Im(n) determines the amount of light, which will beabsorbed in the semiconductor material, while the real part Re(n) of therefractive index determines the speed of light in the medium. Therefractive index is a function of frequency and the amount of absorptionhence depends on the frequency (or wavelength) of the light.

[0010] The lateral confinement of light in typical waveguides is basedon total internal reflection. Total internal reflection is thereflection of EM radiation from the interface of a medium with largerindex of refraction n₁ with a medium of smaller index of refractionn₂<n₁ when making an angle of incidence T! sin¹ $1\frac{n_{2}}{n_{1}}$

[0011] to normal. Thus, the lateral confinement of light depends uponthe index contrast between the waveguide core and the surroundingmaterial as well as upon the angle of incidence of light on theboundaries between the waveguide core and the surrounding material.Hence a change in the index contrast may, depending on the angle ofincidence, introduce losses du to lack of total internal reflection.Also, varying the direction of the lateral confinement parts or sidewalls will change the angle of incidence and may, depending on the indexcontrast, introduce large losses due to lack of total internalreflection. Variations in the direction of the lateral confinement partsmay be a bent waveguide if both sides of the lateral confinement varyidentically. It may also be a variation of only one of the sides such asa narrowing of the waveguide. Alternatively, the width of the waveguidemay vary in that both sides performs repeated change of directions,such, as a wobbling. All these different scenarios will introduce lossessince they change the angle of incidence on at least one side of thelateral confinement boundary of the waveguide, the collective termbending losses will be used for simplicity.

[0012] The present invention provides an optical intensity modulation byintroducing a modulated loss governed by a modulation of the real partof the refractive index. The invention may be implemented as an opticalmodulator based on these modulated losses alone, or may be used toimprove the performance of existing optical modulators by introducing anextra loss for improving the extinction ratio. A modulation of the realpart of the complex refractive index can be accomplished in differentways, however, in order to obtain modulation speeds fast enough forindustrial application in optical communication and related fields, theworking principle and material composition of devices must be carefullyconsidered. The present invention will provide more compact opticalintensity modulators with improved performance (extinction) and speed.

[0013] It is known from prior art electro-absorption modulators to useQCSE to obtain a modulation in the absorption (or equivalently theimaginary part of the refractive index). The modification of therefractive index, shifted due to the QCSE in case of a quantum wellsemiconductor structure comprising an optical active semiconductingmaterial core, is not only restricted to the imaginary part of therefractive index. Also the real part of the refractive index will bemodified. The change of the real part can be calculated from the changesin the imaginary part of the refractive index by the Kramers-Kronigtransformation. In general, QCSE provides a very fast and precisevariation of the real part of the refractive index, however, the QCSEcan only change the real part a small amount (the effective refractiveindex can be changed on the order of a few 10⁻³). Thus, the effectiverefractive index contrast as well as the bandgap frequency of thebandgap is modulated by the electrical field applied to induce QCSE,also referred to as the voltage swing or ∃V.

[0014] The complex refractive index modulations may also be induced byphoto generated charge carriers. In this case, the modulation of thecomplex refractive index is a result of different effects following fromthe absorption of an EM field such as an intensity modulated opticalcontrol signal. The control signal should have a frequency above thebandgap of the optical active semiconducting material in order to beabsorbed and generate free charge carriers in the material. If theoptical active semiconducting material core is electrically biased, thephoto-induced free charge carriers will screen the bias field andthereby modulate the bandgap energy according to the QCSE, hence theterm optically induced QCSE. Furthermore, the photo-induced free chargecarriers affect the complex refractive index of the material, resultingin the desired index modulations.

[0015] The present invention fulfil the objects given above by, in afirst aspect, providing an optical modulator for modulatingelectromagnetic (EM) radiation having a first frequency Θ₁, said opticalmodulator comprising a first waveguide section for guiding the EMradiation, said waveguide section comprising an elongated core regionwith complex refractive index n_(core) having side walls to asurrounding region with complex refractive index n_(surr), thedifference between the real part of n_(core) and the real part ofn_(surr) defining a refractive index contrast∃n=Re(n_(core))−Re(n_(surr)) and at least one of the side walls of thecore region being in the longitudinal direction of the core region, andcomprising means for applying a modulated first and second electric orEM field E₁ and E₂ to the core region, wherein the core region comprisesan optical active semiconducting material having a predeterminedmaterial composition and having an energy bandgap, said energy bandgapbeing positioned at a first bandgap frequency Θ_(bandgap E1) in responseto the application of the first field and being positioned at a secondbandgap frequency Θ_(bandgap E2) in response to the application of thesecond field, n_(core) depending upon the energy bandgap so that thematerial composition provides, for EM radiation of the first frequency,a first complex refractive index n_(core E1) in response to theapplication of the first field and a second complex refractive indexn_(core E2) in response to the application of the second field, andwherein the predetermined material composition and the first frequencyare chosen so that a difference in the index contrasts∃n_(E1)=Re(n_(core E1))−Re(n_(surr)) and∃n_(E2)=Re(n_(core E2))−Re(n_(surr)) results in bending losses for EMradiation of the first frequency guided in the waveguide.

[0016] Thus, according to the first aspect, the index contrast for awaveguide section having an optical active semiconducting material coremay be modulated electronically or optically. If at least one side ofthe lateral confinement boundary of the waveguide is bend, themodulation of the index contrast will result in losses due to lack oftotal internal reflection. Naturally, the bend must be in thelongitudinal direction of the core region so as to intercept thestraight propagation of light in the waveguide. These losses mayefficiently improve the performance of existing absorption-modulators asdescribed in the following.

[0017] As mentioned previously, QCSE can only change the real part ofthe refractive index of an optically active semiconductor material asmall amount, typically on the order of a few 10⁻³, similarly, the shiftin the index contrast due to QCSE induced by the applied fields will becorrespondingly small resulting in a small extinction due to bendingloss modulation. Thus, in order to induce high extinction ratios in themodulated bending losses, the waveguide index contrast should be smallat least in directions in the plane of the bend (transverse directions).Therefore, the index contrast between the core region and thesurrounding regions in the lateral direction is preferably equal to orsmaller than a few 10⁻² such as equal to or smaller than a few 10⁻³.

[0018] By modulating the bandgap frequency of the bandgap of anoptically active semiconductor material, the absorption coefficient willalso be modulated whereby some absorption will occur in the material.However, the absorption of the EM radiation generates free chargecarriers in the active material. These free charge carriers may screenthe applied electrical field and are a part of the circuit performingthe modulation, hence, the transporting of these free charge carrierswill effectively limit the modulation speed. At high light intensities,the generated free charge carriers may ultimately saturate the circuitperforming the modulation.

[0019] However, since the radiation in the waveguide will experiencelarge bending losses as well, the amount of absorbed photons will bereduced, thereby also reducing the generation of free charge carriers.Thus, by combining the absorption modulation scheme with bending losses,the combined extinction from absorption and bending losses will increasethe obtainable modulation speed and extinction. The combined extinctionfrom absorption and bending losses will enhance the extinction ratio ofthe modulator for a given amplitude in the applied fields, voltage swingif an electrical signal is applied and modulation depth if an EM signalis applied.

[0020] As mentioned in the above, in order to induce high extinctionratios in the modulated bending losses, the waveguide index contrastshould be small at least at positions where the bending losses issupposed to take place. Therefore, the waveguide type is preferably aweakly index guided waveguide such as a ridge waveguide or a BuriedHeterostructure (BH) waveguide. The waveguide typically comprises anumber of different material layers deposited on a substrate, thedifferent layers forming a structure which define the core region in onetransverse (typically vertical) direction. At least one of the materiallayers forming the core region is an optically active semiconductormaterial meaning that it has an energy bandgap above which the materialcan absorb photons.

[0021] The means for applying the fields are preferably one or moreelectrical contacts for forming an electric field, the contacts beingformed by one or more electrically conducting material layers depositedon the waveguide structure. Forming and contacting such contacts arewell known techniques within the field of planar waveguides.

[0022] Alternatively, the applied field is an EM field. In this case,the means for applying the first and second fields comprises one or moreoptical input ports for receiving an EM signal of a second frequency andmeans for guiding said signals to the core region. The optical activesemiconducting material and the second frequency of the EM radiationshould be chosen so that the radiation is absorbed.

[0023] In one preferred type of waveguides, the horizontal transverseboundaries are defined by an electric field applied over only part ofthe active layer so as to induce an index contrast in horizontaltransverse direction of the active layer. Alternatively, the core regionis defined in the horizontal transverse by a material region having aslightly different refractive index. Optionally, the lateral confinementis provided by a combination of these effects.

[0024] As can be seen from FIG. 1, the bandgap is not defined by asingle frequency, rather there is an energy boundary region wherein theabsorption increases for increasing energies. By choosing a givenabsorption within this boundary region, it is possible to definecorresponding bandgap frequency, Θ_(bandgap), at which the bandgapstarts, and above and below which one refers to above bandgap or belowbandgap. It is important to stress that the bandgap frequency can bechosen anywhere in the boundary region, and since one often works in theboundary region, a frequency being higher than the bandgap frequencysimply means that light having this frequency experiences a higherabsorption than light having a frequency lower than the bandgapfrequency. Hence a given frequency in the boundary of FIG. 1 may beabove bandgap in a first situation (where it is compared to an evenlower frequency experiencing a smaller absorption) whereas it will bebelow bandgap in another case (compared to a slightly higher frequencyexperiencing a higher absorption). Therefore, it is generally notpossible to assign a specific bandgap frequency to one of the curves inFIG. 1, as it depends on the specific situation.

[0025] Preferably, the bending losses are introduced by applying a bentwaveguide section. Alternatively, the waveguide is designed so that atleast one of the side walls is bent so as to vary the width of thewaveguide. Such design will also introduce bending losses due to themany small bends in the sections. Also, different types of bendinglosses may be combined.

[0026] The combination of absorption and bending losses enhance theextinction ratio for a given amplitude of the applied field. The effectthat the bending losses reduce the amount of free charge carriers leadsto a number of advantages, most important an increased modulation speedand reduced tendency for the free charge carriers to saturate themodulation circuit. These effects make the modulator more efficientwhich allows a decrease in the size of the modulator while keeping theefficiency (extinction ratio) constant. The increased efficiency of theextinction also allows for smaller and thereby faster electrodes forapplying the field.

[0027] Depending on the exact purpose and design of the waveguide, theratio between the contributions from the two means of extinction,absorption and bending losses, may be varied. If the optical signal tobe modulated, i.e. the EM radiation of the first frequency, has afrequency below the bandgap frequency of the predetermined materialcomposition (with the applied field), only modulated bending losses willbe introduced. If the optical signal to be modulated has a frequencyabove the bandgap frequency, the modulation in the imaginary part of therefractive index will introduce modulated absorption losses whereas themodulation in the real part of the refractive index will introducemodulated bending losses. Thus, by controlling the material compositionand the first frequency, the extinction can be precisely controlled.

[0028] Thus, in a first preferred embodiment, the predetermined materialcomposition of the optical active semiconductor material is preferablyadjusted so that, for EM radiation of the first frequency, the firstcomplex refractive index, n_(core E1) and the second complex refractiveindex, n_(core E2) fulfil the relations:

[0029] I. Re(n_(core E1))>Re(n_(core E2)) giving a first refractiveindex contrast ∃n_(E1) if the first field is applied and a secondrefractive index contrast ∃n_(E2) if the second field is applied, thefirst refractive index contrast being larger than the second refractiveindex contrast, ∃n_(E1)>∃n_(E2),

[0030] II. Im(n_(core E1))<Im(n_(core E2)), giving a first bandgapfrequency larger than the first frequency, Θ_(bandgap E1)>Θ₁, inresponse to the application of the first field and a second bandgapfrequency smaller than the first frequency, Θ_(bandgap E2)<Θ₁, inresponse to the application of the second field.

[0031] In this preferred embodiment, when the first field is applied,the index contrast is high and the first frequency is below bandgapresulting in an efficient, low loss guiding through the bent section anda low absorption. Hence, when the first field is applied, both means ofextinction work to give a high transmission through the waveguidesection. When the second field is applied, the index contrast is smallresulting in large bending losses, and the first frequency is abovebandgap resulting in a high absorption. Hence, when the second field isapplied, both means of extinction work to give a large extinction in thewaveguide section.

[0032] This situation is illustrated in FIG. 2 for the case where theapplied field is an electric field. In FIG. 2, wherein the predeterminedmaterial composition of the optical active semiconductor material isadjusted so as for the first frequency to lie in the shaded region. InFIG. 2, the real and imaginary part of the refractive index of thesemiconductor material are given as a function of energy for twodifferent applied fields such as no bias and negative bias correspondingto the first and second field respectively. It can be seen that when thesecond field is applied, the QCSE shifts the values of Re(n_(core)) andIm(n_(core)) to values resulting in increased bending losses andabsorption. Due to the bending losses, the extinction ratio is enhancedand the amount of photo-generated free charge carriers is reduced, botheffects contributing to a more efficient modulation allowing for areduction in size and voltage swing compared to existing EAMs.

[0033] In a second preferred embodiment, the predetermined materialcomposition of the optical active semiconductor material is preferablyadjusted so that, for EM radiation of the first frequency, the firstcomplex refractive index, n_(core E1) and the second complex refractiveindex, n_(core E2) fulfil the relations:

[0034] I. Re(n_(core E1))<Re(n_(core E2)) giving a first refractiveindex contrast ∃n_(E1) if the first field is applied and a secondrefractive index contrast ∃n_(E2) if the second field is applied, thefirst refractive index contrast being smaller than the second refractiveindex contrast, ∃n_(E1)<∃n_(E2),

[0035] II. Im(n_(core E1))|Im(n_(core E2)) resulting in a bandgapfrequency larger than the first frequency if either of the first orsecond field is applied, Θ_(bandgap E1)>Θ₁ and Θ_(bandgap E2)>Θ₁.

[0036] In the second preferred embodiment, the bending losses give themajor contribution to the extinction. When the first field is applied,the index contrast is small resulting in large bending losses, and thefirst frequency is below bandgap resulting in a low absorption. Hence,the extinction results primarily from the bending losses. When thesecond field is applied, the index contrast is high resulting in anefficient, low loss guiding through the bent section, and the firstfrequency is below bandgap resulting in a low absorption. Hence, whenthe first field is applied, only the bending losses work to give a largeextinction, whereas when the second field is applied both effects allowsfor an efficient low-loss guiding in the waveguide section.

[0037] This situation is illustrated in FIG. 3, where the applied fieldis an electric field. In FIG. 3, the predetermined material compositionof the optical active semiconductor material is adjusted so as for thefirst frequency to lie in the shaded region. It can be seen that whenthe second field is applied, the QCSE shifts the values of Re(n_(core))to values resulting in increased bending losses while Im(n_(core)) isvery small and do not change significantly. Typically, Im(n_(core,E1))is slightly smaller than Im(n_(core,E2)) meaning that the absorption islarger when the bending loss is low. Therefore, the predeterminedmaterial composition of the optical active semiconductor material ispreferably adjusted so as for the absorption to be very low when thesecond field is applied. Therefore, no photo carriers are generatedwhich could otherwise cause modulation speed limitations due totransport times.

[0038] When an optical mode propagates in a bent waveguide section, itis shifted toward the outer perimeter of the bend. Therefore, couplingof different waveguide sections is also a source of losses, couplinglosses. It is known, e.g. from Veldhuis et al, that these couplinglosses depend on the index contrast of the coupled waveguide sections.

[0039] Thus, in a third preferred embodiment the optical modulatorfurther comprises a second waveguide section similar to the firstwaveguide section and positioned in extension of the first waveguidesection, said second waveguide section having a coupling to the firstwaveguide section which is adapted to introduce coupling losses forradiation in the optical modulator, said coupling losses depending onthe refractive index contrast in parts of the sections close to thecoupling.

[0040] Hence, when modulating the refractive index to modulate thebending losses, the modulator may be designed to benefit from thecoupling losses' dependence upon the index contrast. Therefore, themeans for applying the first and the second field preferably comprisesmeans for applying the first and the second fields to core regions closeto the coupling in the first and/or second waveguide section, so as tomodulate the refractive index contrast in these regions.

[0041] For all embodiments of the modulator according to the firstaspect of the invention, the material composition of the modulator isvery important for obtaining the required relations between the complexrefractive indices at different applied electric fields. A variety ofwaveguide designs are applicable, and the design parameters includingthe material composition may be very precisely determined using existingsemiconductor processing technologies. Preferably, the core and/or thesurrounding regions are at least substantially formed by one or more ofthe materials selected from the group consisting of III-V or II-VIsemiconductor materials. The III-V material could typically be InP,GaAs, AlGaAs, InGaAsP, whereas a typical II-VI material could be ZnSe.

[0042] Depending on the design, it may be necessary to dope one or morematerial layers, hence the core and/or the cladding region may be dopedwith one or more of the materials selected from the group consisting ofBe, Zn, Mg, Si, C and S.

[0043] An optical modulator according to the present invention mayadvantageously be applied for modulating light signals in order toencode information into the signals. Hence, the means for applying thefirst and the second electric field preferably comprises one or moreelectrical contacts for receiving an electric signal and generating thefirst and second electrical field in response to the received electricsignal. The modulator may further comprise ultra fast receivers andamplifiers for receiving the signal. Typically, the received signal willbe a binary signal and the means for generating the first and secondelectrical field is preferably adapted to generate the first fieldcorresponding to “0” and the second field corresponding to “1”, or viceversa.

[0044] The modulator according to the present invention will preferablybe used in optical communication. Hence the material compositions arepreferably optimised for providing optimum modulation for light having awavelength in the region from 500 nm to 2000 nm. Preferably, themodulator is optimised for light having a wavelength in the region 750nm to 900 nm or 1300 nm to 1650 nm, preferably within smaller regionscentred at 850 nm, 1350 nm or 1550 nm.

[0045] In the case where the applied field is an electric field, thefirst applied electric field is preferably at least substantially zeroand the second applied electric field is negative. This is because theQCSE typically gives the largest shift for negative bias therebyrequiring a smaller voltage swing.

[0046] According to a second aspect, the present invention provides amethod for modulating EM radiation using the optical modulator accordingto the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

[0047]FIG. 1 shows the shift of the absorption bandgap for an opticalactive semiconductor due to the QCSE.

[0048]FIGS. 2 and 3 shows the real and imaginary part of the refractiveindex of an optical active semiconductor material as a function ofenergy for two different applied fields. The shaded regions indicate thepreferred frequencies of the radiation to be modulated.

[0049]FIGS. 4A and B shows a cross sectional, simplified views of atypical ridge type waveguide.

[0050] FIGS. 5 to 9 shows a number of different bent waveguide designshaving different lengths and offsets and applying a different number ofbent waveguide sections having different curvatures.

[0051]FIG. 10 is a top view of an illustration of a bent waveguidemodulator according to a preferred embodiment of the present invention,wherein the modulation is controlled by an intensity modulated opticalsignal.

DETAILED DESCRIPTION OF THE DRAWINGS

[0052] The optical modulator according to the present invention isreferred to as a Bent Electro-Absorption Modulator or BEAM. The basicworking principle of the BEAM modulator is related to the behaviour ofthe complex material refractive index of the semiconductor material.

[0053] The modification of the refractive index due to a reverse bias asknown from QCSE is not only restricted to the imaginary part of therefractive index. Also the real part of the refractive index will bemodified. The change of the real part can be calculated from the changesin the imaginary part of the refractive index by the Kramers-Kronigtransformation. The real part refractive index changes may result in amodulation of the light field as will be described below.

[0054] In general, the QCSE can only change the effective refractiveindex a small amount (on the order of a few 10⁻³). Therefore, thedifference between the core and the cladding index in the horizontaldirection must also be of the same order of magnitude. Thus, thewaveguide type is typically a ridge waveguide or a BuriedHeterostructure (BH) waveguide, since both types can be made to weaklyguide the light.

[0055] As mentioned previously, the complex refractive index may also bemodulated optically through the QCSE by mechanisms analogue to the casewith electrical fields. Most of embodiments will in the following willbe described in relation to the case with electrically induced QCSE, butare equally applicable with optically induced refractive indexmodulations by induced by photo generated carriers.

[0056]FIG. 4 shows a weakly index guiding ridge waveguide. The shownridge waveguide consist of a first cladding layer 20, an active layer 22and a second cladding layer 24. Cladding layers are typically Zn dopedInP whereas the active layer typically consist of alternating layers ofGaInAsP lattice matched to InP with alternating bandgap depending oncomposition. The low bandgap layers are named Quantum Wells since theyform low energy holes which bind the charged carriers. The effectiverefractive index for the core layer 22 and cladding layers 20 and 24 aretypically 3.216 and 3.204, respectively, and the cladding layersprovides a transverse confinement in the waveguide. On top of claddinglayer 24 are three regions 28 and the region 26 defining the lateralconfinement of the waveguide. The region 26 consists of Zn-doped InP andforms the ridge of the waveguide. The region 28 consists of aplanerising polymer with very low optical material absorption and arefractive index significantly below that of region 26. Thus, thedifferent refractive indices in the three regions form the lateralwaveguiding. The core region is the area where the refractive index maybe modified when applying a reverse bias to the semiconductor using goldelectrodes 30 and 32, and is illustrated by the rectangle 34 in FIG. 4A.The waveguide is conducting light, the optical mode 36 will bedistributed through the core and cladding layers as illustrated in FIG.4B.

[0057] By changing the real part of the refractive index of the core,the lateral guiding and thus the waveguide loss may be modified. Theloss of a waveguide wherein only the lateral optical confinement ismodified may be significantly affected by using bent waveguide sections.The bends may be either cosine shaped bends, bends with constantcurvature etc. Furthermore, combining the bend with a modulation of thewaveguide lateral width will increase the waveguide losses further.Typical waveguide bent designs are shown in FIGS. 5 through 9.

[0058]FIG. 5 shows a waveguide with 6 sections: a straight waveguide, 4bent waveguides and a straight waveguide section. The vertical linedenoted the interface between the sections. The ridge width is typically2-3 Πm and the off-set at the centre of the bent structure is typically4-16 Πm. The length of the waveguide is typically 100-400 Πm. Shortbends, and thus small area electrical contacts are preferable withrespect to high modulation speeds. An extinction ratio typical of atleast 8 to 20 dB is possible with an appropriate design and an electricfield induced effective refractive index variation of a few 10⁻³. Thecoupling loss between sections can be modified and become more sensitiveto refractive index changes by displacing the different sections by afraction of the waveguide width, typically 1 Πm.

[0059]FIG. 6 shows 2 bent waveguides placed close to another. Thewaveguide below will be able to capture a fraction of the light leaking(bending loss) from the upper waveguide structure. The waveguide thusacts as an aperture, preventing re-capture of the lost or leaked light.

[0060]FIG. 7 shows a bent structure where the input and output waveguidesections are laterally displaced, which can be advantageous with respectto avoiding recapture of lost or leaked light.

[0061]FIG. 8 corresponds to FIG. 7, except that the output waveguide isangled, minimising reflections from the cleaved facets.

[0062]FIG. 9 shows a combination of bent waveguide sections and amodulation of the distance from the centre to the sides of thewaveguide. This modulation on either side of the waveguide being eitherin phase (“wiggle”) or out of phase (“wobble”). All these waveguidemodifications have effects on the waveguide properties and thesensitivity to the applied field.

[0063] Spectrally, there are two working regions for the BEAM, marked bythe shaded regions in FIGS. 2 and 3. The shaded regions indicate thepreferred frequencies of the radiation to be modulated.

[0064] In the first working region 4, shown in FIG. 2, the operationpoint is above-bandgap. In this case the bent waveguide section willwork like an ordinary EAM, but in a particular wavelength region theworking principle of the EAM will be improved, since the bend-lossesenhance the modulation response. In FIG. 2 the shaded region 4 denotes aregion where the absorption is low when no bias is applied.Simultaneously the index of the core is high resulting in low bendinglosses. Increasing the reverse bias the QCSE shifts the absorptionspectra to lower energies resulting in a high absorption. The refractiveindex of the core region decreases with reverse bias, resulting inlarger bend-losses. The modulator works in this case both as an EAM andas a bent waveguide modulator.

[0065] In the second working region 6, shown in FIG. 3, the workingpoint is below bandgap and the waveguide structure may be transparent(zero or low absorption). In case no bias is applied the solid curve hasa low refractive index and the waveguide will have a high bend-loss.Increasing the reverse bias will result in an increase of the coreeffective index and thus a reduction of the bend-losses. A smallincrease in absorption may also be observed due to the QCSE, but thismay be minimised by a proper design of the semiconductor structure.

[0066] There are a number of key material and design parameters toconsider when designing a BEAM modulator, most of which are relevant fora BEAM working in either of the regions 4 and 6 of FIGS. 2 and 3respectively. These will be described in the following.

[0067] The sensitivity of the material to the applied electric field andthus the resulting shifts in both the real and imaginary part of therefractive index as a function of applied electric fields depend onseveral epitaxial growth related issues, such as quantum well materialcomposition, energy depth of wells, well widths, distance between p andn-doped layers, etc. By designing the epitaxial, active core material tohave a large refractive index change as a function of electric field, anumber of restrictions in design can be relaxed. For example, thewaveguide can be designed to be less sensitive to refractive indexchanges and thus be less sensitive to fabrication variations. Also, byoptimising the electric field sensitivity of the material, theextinction ratio can be optimised and the demands for the magnitude ofthe applied field can be relaxed.

[0068] The refractive index contrast between the core and claddingregions must be tuned to a value appropriate to the corresponding shiftof the real part of the refractive index of the core region. If theindex contrast is too small, the resulting guiding will not besufficient to keep the light from leaking to the surroundings. Also thefabrication tolerances will be tight. If, on the other hand, the indexcontrast is too large, the index changes due to the applied electricfield will not be sufficient to significantly alter the guiding.

[0069] The sensitivity of the efficiency of the guiding depends on themode size and thus on the cross sectional dimensions of the core region.Hence, the width and height of the waveguide have a large effect of thelosses in the bent waveguide sections as well as in couplings. Bydesigning the waveguide to have local variations in the cross sectionalshape, such as to “wiggle” or “wobble” as shown in FIG. 9, certaindesirable guiding features may be optimised.

[0070] The curvature of the waveguide naturally needs to be such thatthere is some leakage which may be modulated by the applied field. Thecurvature may, as previously mentioned, be constant, or vary accordingto some function. Typically, the bend will comprise a number of smallerbent waveguide sections, which may have different curvatures and bendingdirections. Hence the curvature of the waveguide may be very complex.Calculations have shown that the way in which each section is interfacedto the next may have a large effect on the guiding properties, thus thecoupling between sections is an important design parameter as well.

[0071] The length of each waveguide section will have a large effect onthe amount of transmitted light, since the total bending losses andabsorption depends upon how long a path the radiation has experiencedthese effects. Hence, the length of the waveguide will have a largeeffect on the amount of transmitted light both in the “on” and “off”state since there is always some bending losses and absorption, even inthe “off” state. Light which has been lost or leaked may be recapturedin the waveguide since the waveguide may bend back into the path of thelost or leaked light. The angle between waveguide and output facet isanother design variable which may be optimised to minimise recapture oflight.

[0072] Some further material and design parameters are only relevant fora BEAM working in either of the region 4 of FIG. 2, in which absorptionis important.

[0073] In some cases it may be advantageous to maximise the ratiobetween material absorption at the two applied electrical fields inorder to maximise the extinction ratio. In other cases, it is moreimportant to balance the absorption and bending losses in order tomaximise device speed.

[0074] Clearly, high transmission is important for the “on” state inorder to minimise insertion losses. Thus, in the “on” state of thedevice, the material absorption may be made as low as possible withoutjeopardising the material absorption in the “off” state.

[0075] A sample of a BEAM has been realised for testing the workingprinciple of the present invention. Our test devices were made of III-Vmaterial, InGaAsP, which were lattice matched to InP. The active regioncomprised of 10 quantum wells, which had a band gap such that thephotoluminescence peaked around 1555-1557 nm. The waveguides were of theridge waveguide type, where the effective refractive index of the coreand cladding was estimated to be 3.216 and 3.204, respectively.

[0076] The test devices comprised of 3 sets of devices, each setcomprising of 8 devices with different curvatures for the bent sections.Each device comprised of two sections, a straight waveguide and a curvedwaveguide such that the total waveguide length was approximately thesame for all the devices. For each device the bent section had adifferent curvature. In all cases, the length and width of the waveguidewas 1 mm and 3 Πm, respectively and the offset was 65 Πm. The curvaturewas determined by the length of the curved section, which varied between500 Πm and 1000 Πm in steps of 100 Πm.

[0077] The devices were fitted with electrical contacts above and belowthe waveguide, which made it possible to apply a negative electric fieldacross the active region (across the quantum wells). The devices werethen mounted to a copper heat sink for temperature control.

[0078] In testing the devices, the light from a laser was coupled intoone device at the time and the transmission of this light as a functionof the magnitude of the reverse bias was measured. The wavelength of thelaser light was 1593 nm, which meant that both coupling losses as wellas absorption were present (energy region 2). By comparing theperformance of the bent devices with the straight it was possible tosubtract the contribution from the absorption and thus being able toassess how the light propagation through the bent sections was affectedby the change of the refractive index.

[0079] When using a bent waveguide for modulation purposes at typicalcommunication frequencies, the length of the waveguide would most oftenbe much shorter than for the test device, typically of the order of100-200 Πm, in order to increase device speed. Also, a number of theother features discussed under the material and design key parameterswould be introduced.

[0080]FIG. 10 illustrates another preferred embodiment of the presentinvention, namely an optical modulator 40 using optically induced QCSE.The modulator 40 comprises an input port 42 for receiving an EM signal41 to be modulated having a first frequency Θ₁. From the input port 42,the signal 41 is guided by a waveguide section 43 to the bent waveguidesection 44. The modulator further comprises an input port 46 forreceiving a modulating signal 45 having a frequency Θ₂. The signal 45 isguided by a waveguide section 47 to the bent waveguide section 44.Sections 43 and 47 together establishes a coupler for superposing thesignals 41 and 45 in the bent waveguide section 44. The modulatingsignal 45 modulates the complex refractive index, Re(n_(core)) andIm(n_(core)), of the optical active semiconductor material core of thebent waveguide section 44.

[0081] The modulation in Re(n_(core)) results in a modulation in theindex contrast between the core material of the bent waveguide sectionand the typically constant refractive index of the surrounding material.Since the bending losses for the signal 41 depends on the indexcontrast, modulated bending losses 48 is induced in signal 41 if signals45 and 41 overlaps temporarily. As a result, a resulting, modulatedsignal 49 has the same shape as the modulating signal 45 but thefrequency Θ₁ of the signal 41.

[0082] The modulation in Im(n_(core)) may, depending on which of thescenarios described in relation to FIGS. 2 and 3 is chosen, result in amodulation of the absorption of the signal 41 in the core material ofthe bent waveguide section 44. Thereby, radiation guided by the bentwaveguide section 44 despite the bending loss may be absorbed. Thiscombined extinction, bending losses and absorption, improve theextinction ratio and/or the modulation depth of the resulting signal 49.

[0083] The BEAM has a large number of potential applications, some ofwhich are listed below.

[0084] 1. Modulation in general. The BEAM may be used to directlymodulate light. For very high speed modulation, it could be advantageousto operate in regime 1, where no material absorption takes place. Inthat case no carriers have to be removed from the active region to avoida pile-up effect that will reduce the applied voltage. The modulationcould for example be a high bitrate bitstream.

[0085] 2. Time-demultiplexing of OTDM channel. By keeping the BEAM inthe “off” state expect for short “window openings” at a specificfrequency, the BEAM may be used to multiplex out one time slot in anOptical Time Division Multiplexed (OTDM) bitstream.

[0086] 3. Cross Phase Modulation for wavelength conversion. One maychange the refractive index through EM radiation as well as an appliedelectric field. By coupling a modulated, high intensity pumping lightwith a photon energy above bandgap through the BEAM, material absorptiontakes place. The generated carriers will change the refractive index andthus the guiding properties. By simultaneously incoupling light with aphoton energy below bandgap, the change in guiding properties willmodulate the light in a pattern being either the same or the inverse ofthe pattern of the pump light.

[0087] 4. Clock generation. By simply applying a voltage which issinusoidal as a function of time, the BEAM may generate an opticalsignal which may be used as an optical clock for communication systems.

[0088] 5. Part of multiplexing system, where the multiplexing is doneelectrically. As in 1. the BEAM could be used in a system which as inputhas several electrical signals and as output has one single opticalsignal. The electrical signals will be multiplexed together electricallyand the fast, multiplexed signal is applied to the BEAM and used tomodulate light.

[0089] 6. Part of signal regeneration system. As part of a signalregeneration system, electronics will detect, reshape and amplify theincoming signal and apply the resulting voltage to the BEAM so that theoutput light is an improved copy on the input.

[0090] 7. All-optical demultiplexing. As in 2. the BEAM could be kept inthe “off” state expect for short “window openings” at a specificfrequency were an optical control pulse is injected above bandgap tocreate a change in the effective refractive index below bandgap asdescribed in 3.

[0091] In all the above examples, the BEAM may be used to replacetraditional EAs where very high speed or better extinction ratios areneeded. Since there is reduced or no material absorption involved in themodulation process, high speed limitations related to carrier transporthave been removed/reduced.

1. An optical modulator for modulating electromagnetic (EM) radiation having a first frequency Θ₁ said optical modulator comprising a first waveguide section for guiding the EM radiation, said waveguide section comprising an elongated core region with complex refractive index n_(core) having side walls to a surrounding region with complex refractive index n_(surr), the difference between the real part of n_(core) and the real part of n_(surr), defining a refractive index contrast ∃n=Re(n_(core))−Re(n_(surr)) and at least one of the side walls of the core region being in the longitudinal direction of the core region, and comprising means for applying a modulated first and second electric or EM field E₁ and E₂ to the core region, wherein the core region comprises an optical active semiconducting material having a predetermined material composition and having an energy bandgap, said energy bandgap being positioned at a first bandgap frequency Θ_(bandgap E1) in response to the application of the first field and being positioned at a second bandgap frequency Θ_(bandgap E2) in response to the application of the second field, n_(core) depending upon the energy bandgap so that the material composition provides, for EM radiation of the first frequency, a first complex refractive index n_(core E1) in response to the application of the first field and a second complex refractive index n_(core E2) in response to the application of the second field, and wherein the predetermined material composition and the first frequency are chosen so that a difference in the index contrasts ∃n_(E1)=Re(n_(core E1))−Re(n_(surr)) and ∃n_(E2)=Re(n_(core E2))−Re(n_(surr)) results in bending losses for EM radiation of the first frequency guided in the waveguide.
 2. An optical modulator according to claim 1, wherein the means for applying the first and second fields comprises one or more electrical contacts for receiving an electric signal.
 3. An optical modulator according to claim 1, wherein the means for applying the first and second fields comprises one or more optical input ports for receiving an EM signal having a second frequency and means for guiding said signals to the core region, and wherein the optical active semiconducting material absorbs EM radiation of the second frequency.
 4. An optical modulator according to claim 1, wherein the core and/or the surrounding region are at least substantially formed by one or more materials selected from the group consisting of III-V and II-VI semiconductor materials.
 5. An optical modulator according to claim 4, wherein the core and/or the cladding region is doped with one or more of the materials selected from the group consisting of Be, Zn, Mg, Si, C and S.
 6. An optical modulator according to claim 1, further comprising a second waveguide section in extension of the first waveguide section, said second waveguide section having a coupling to the first waveguide section which is adapted to introduce coupling losses for radiation in the optical modulator, said coupling losses depending on the refractive index contrast in core regions adjacent to the coupling.
 7. An optical modulator according to claim 6, wherein the means for applying the first and the second field comprises means for applying the first and the second fields to core regions close to the coupling in the first and/or second waveguide section, so as to modulate the refractive index contrast in these regions.
 8. An optical modulator according to claim 1, wherein the first applied field is at least substantially zero.
 9. An optical modulator according to claim 1, wherein the predetermined material composition of the optical active semiconductor material is adjusted so that, for EM radiation of the first frequency, the first complex refractive index, n_(core E1) and the second complex refractive index, n_(core E2) fulfil the relations: I. Re(n_(core E1))>Re(n_(core E2)) giving a first refractive index contrast ∃n_(E1) if the first field is applied and a second refractive index contrast ∃n_(E2) if the second field is applied, the first refractive index contrast being larger than the second refractive index contrast, ∃n_(E1)>∃n_(E2), II. Im(n_(core E1))<Im(n_(core E2)), giving a first bandgap frequency larger than the first frequency, Θ_(bandgap E1)>Θ₁, in response to the application of the first field and a second bandgap frequency smaller than the first frequency, Θ_(bandgap E2)<Θ₁, in response to the application of the second field.
 10. An optical modulator according to claim 1, wherein the predetermined material composition of the optical active semiconductor material is adjusted so that, for EM radiation of the first frequency, the first complex refractive index, n_(core E1) and the second complex refractive index, n_(core E2) fulfil the relations: I. Re(n_(core E1))<Re(n_(core E2)) giving a first refractive index contrast ∃n_(E1) if the first field is applied and a second refractive index contrast ∃n_(E2) if the second field is applied, the first refractive index contrast being smaller than the second refractive index contrast, ∃n_(E1)<∃n_(E2), II. Im(n_(core E1))|Im(n_(core E2)) resulting in a bandgap frequency larger than the first frequency if either of the first or second field is applied, Θ_(bandgap E1)>Θ₁ and Θ_(bandgap E2)>Θ₁.
 11. An optical modulator according to claim 1, wherein the waveguide section is adapted to introduce bending losses in that the waveguide section is a bent waveguide section.
 12. An optical modulator according to claim 11, wherein the waveguide section is adapted to introduce bending losses in that the waveguide section comprises two or more small bends.
 13. An optical modulator according to claim 1, wherein the waveguide section is adapted to introduce bending losses in that the width of the waveguide section is varied.
 14. An optical modulator according to claim 1, wherein the first frequency of EM radiation to be modulated has a wavelength within the region from 500 nm to 2000 nm, such as within the region 750 nm to 900 nm or 1300 nm to 1650 nm.
 15. An optical modulator according to claim 1, wherein the first frequency of EM radiation to be modulated has a wavelength within an interval centred at 850 nm, 1350 nm or 1550 nm, said interval having a width of 50 nm.
 16. A method for modulating EM radiation using the optical modulator according to claim
 1. 